A frequency offset occurring in a mobile communication system includes two types. One is the frequency offset generated by a Carrier Frequency Offset (CFO) between a terminal and a base station, and the other is the frequency offset caused by Doppler frequency.
The terminal synchronizes with a transmit carrier frequency of the base station through initial ranging or periodic ranging. When the terminal cannot achieve an accurate synchronization or when the accurate synchronization is compromised by movement of the terminal, a frequency offset takes place. Also, when the terminal transmits signals during the fast movement, the Doppler frequency can cause a frequency offset.
Such a frequency offset increases channel change with time of the received signal, grows channel estimation error, and degrades an overall performance of the whole modem.
When the terminal synchronizes and transmits with the carrier frequency including the frequency offset in the downlink, the frequency offset of a base station modem is double a maximum Doppler frequency.
An estimation range using a pilot signal of a Physical Resource Unit (PRU) is determined by how many symbols a pair of pilots of the same subcarrier are separated from each other in the time domain. Disadvantageously, the estimation range cannot cover a maximum range of the frequency offset occurrence.
In addition, when the terminal is traveling fast, it is hard to estimate the frequency offset merely with the pilot signal.